When calibrating or testing Off-limits or transducer with a pneumatic pressure controller you should protect the controller from any liquid contaminants that may be present in the device under test (DUT). A block and bleed valve (BBV) in-line with a coalescing filter is a superb way to do this.
The potential flow of containments is from the DUT to the controller. A BBV and a coalescing filter, oriented as observed in the diagram above, will to work to avoid contamination. The BBV can be used to purge the majority of liquid from the DUT. The coalescing filter is really a backup to remove any residual liquid that may be present even after the original purge and will separate entrained liquid droplets from gas flowing in the direction of the controller. The next procedure is built to prevent damage to a pneumatic pressure controller from a contaminated DUT.
Purge the DUT of liquid containments
Before connecting the DUT to the system, remove just as much liquid as you possibly can from the DUT.
With the controller power off and both block and bleed valve closed, connect the machine as shown above.
Use the controller to use a pressure close to the full scale pressure of the DUT.
Open the block valve slowly to pressurize the DUT (note: the gas will flow in direction of the controller to the DUT ? high pressure to low pressure).
Following Should is pressurized close the block valve.
Open the bleed valve and purge the system to atmospheric pressure. Take notice of the sump to see if any liquid is purged.
Close the bleed valve.
Repeat steps 3 to 7 until forget about liquid is observed coming out of the system.
With the block and the bleed valves securely closed, vent the controller.
After completion of the above procedure it will now be safe to calibrate the DUT. To calibrate, open the block valve and close the bleed valve. The coalescing filter will coalesce (join together) any liquid droplets that may still be present in the gas, and gravity will draw the liquid to underneath of the filter housing. The filter may also remove particles larger than the specified size. A computerized or manual valve can be attached to underneath of the filter housing to periodically drain any liquid which has accumulated.
Note:
1. The coalescing filter won’t protect against a big slug of liquid that fills the housing and is forced through the filter membrane or liquid vapor.
2. The gas used to calibrate the DUT can flow in either direction within this system.
3. You will have a pressure drop across the filter in the flowing condition; at steady state pressure there is no pressure drop.